Pearlescent or luster pigments are used in automotive coatings, decorative coatings, plastic articles, paint, printing, inks, cosmetics and the like.
Pearlescent pigments based on laminar substrates such as mica which have been coated with a metal oxide are known. As a result of reflection and refraction of light, these pigments exhibit a pearl-like luster. Also depending on the thickness of the metal oxide layer, they can also exhibit interference color effects. A description of such pigments can be found, for example, in U.S. Pat. Nos. 3,087,828 and 3,087,829.
The pearlescent pigments most frequently encountered on a commercial basis are titanium dioxide-coated mica and iron oxide-coated mica pearlescent pigments. The oxide coating is in the form of a thin film deposited on the surface of the mica particle. The resulting pigment has the optical properties of thin films and thus the color reflected by the pigment arises from light interference which is dependent on the thickness of the coating.
Other metal oxide coatings and/or laminar substrates can also be employed. For example, U.S. Pat. No. 5,753,371 describes pearlescent pigments which are metal oxide-coated glass. Similarly, luster pigments in which a metal or metal alloy such as silver, gold and silver-gold alloys are coated on a laminar substrate such as mica or titanium dioxide-coated mica or other substrates are described in U.S. Pat. No. 5,624,486.
All such pigments achieve their optical effect based on the directed reflection and refraction of light at predominantly sheet-like, mutually parallel-oriented, metallic or reflective pigment particles. As a result, the quality of a pigment is generally dependent on the smoothness or continuousness of the coating on the substrate particle. The quality of the pigment will decrease rapidly with increasing the discontinuities in the coating. For example, silver and/or gold coatings on a particulate laminar substrate have been found to preferentially form blade-shaped metal crystals which overlap with one another on the exterior surface of the coating. As a result, the quality of the pigment suffers even if some non-decorative quality may be enhanced.
In addition, the formation of metal oxide-coated mica (or other substrates) as well as the formation of the silver and/or gold coatings on the substrates often involve the use of a material containing chlorine. For example, formation of a titanium dioxide-coated mica (for use as a substrate on which another layer is formed) often involves the use of titanium tetrachloride as a reagent and/or control of pH during the process through the use of hydrochloric acid. It has been found that the presence of even very small concentrations of aqueous chloride cause the silver and gold coatings on a particulate substrate to disintegrate. The possible presence of residual chlorine means that care must be taken in completely removing reagents employed in the process and/or the resulting silver/gold coated pigments must be maintained in a non-aqueous environment.
It is accordingly the object of the present invention to provide a product in which the silver or gold coating on a laminar substrate is substantially smooth and continuous or the product contains a barrier which protects the coating from degradation by aqueous chloride or, preferably, both. These and other objects of the invention will become apparent to those of ordinary skill in this art from the following detailed description.
The present invention relates to an improved noble metal coated laminar substrate in which the coating contains a small amount of rhodium and/or there is a silane overcoating on the noble metal coating.